1. Field of the Invention
The present invention relates to a driving method of a liquid drop ejecting head for discharging liquid drops to record an image.
2. Description of the Related Art
As liquid drop ejecting heads using an electromechanical transducer (such as piezoelectric actuator), there conventionally exists ink-jet recording heads for discharging ink drops onto recording sheets to record images.
In such an ink-jet recording head of this kind, if a driving waveform is applied to an electromechanical transducer, the meniscus action of a nozzle can be precisely controlled and by doing so, it is possible to undertake high frequency ejection, to eject microdrops, and to prevent satellite drops and mist from being generated. The driving waveform to be applied to the electromechanical transducer is set in accordance with ejection efficiency of the head (ejector) and pressure wave natural oscillation period (Helmholtz oscillation period (Tc)).
In recent years, for ink-jet recording heads, in order to reduce bleeding of ink on the recording sheet and to realize high quality image recording and double-sided printing, the need for liquid drop ejecting heads capable of ejecting high viscosity ink is increasing. In industrial uses other than image recording, if high viscosity liquid can be ejected, there is a merit that the application range of an apparatus can be greatly enlarged.
When high viscosity liquid is to be ejected from an liquid drop ejecting head, however, various problems occur. For example, when such high viscosity liquid is ejected, ejection efficiency of an ejector and refilling ability of the ejector are deteriorated due to the influence of the viscosity of the ink, it is necessary to increase the size of the pressure chamber, and to design flow paths such that fluid resistance is reduced in the nozzle and liquid supply paths.
In the case of high viscosity liquid, the amount of variation in viscosity caused by environmental temperature changes is also increased. For example, in the case of low viscosity liquid having a viscosity of 3 mPa·s, viscosity variation range in the environmental temperature of 5 to 40° C. is about 1.5 to 6 mPa·s, but in the case of high viscosity liquid having viscosity of 20 mPa·s, the viscosity variation range is increased as high as about 10 to 40 mPa·s. Such a large variation of viscosity caused by the environmental temperature has a large effect on the ejection characteristics of an ejector. That is, ejection efficiency of the ejector varies greatly with the environmental temperature. Large variation is generated also in the natural oscillation period Tc of pressure wave, and when the environmental temperature is varied to low or high temperature, there is a problem that liquid drop cannot be ejected normally.
To eliminate the variation in environmental temperature, conventionally proposed is to heat the entire liquid drop ejecting head by a heater, and to keep the ink in the heat at a constantly high temperature (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 11-170515).
There is also proposed a technique for varying driving waveform in accordance with the environmental temperature. For example, JP-A No. 11 -170522 describes a technique in which the voltage of the driving waveform is varied in accordance with the variation in environmental temperature, JP-A No. 2002-326357 describes a technique in which the pulse width of a rectangular driving waveform is varied in accordance with the variation in environmental temperature, and JP-A No. 10-151770 describes a technique in which the time interval of a portion of the driving waveform is varied in accordance with the environmental temperature.
According to the technique described in the above JP-A No. 11-170515, however, there is a problem that it leads to the apparatus size and cost being increased, and it takes a long time to warm up the apparatus.
As to the techniques of JP-A Nos. 11-170522, 2002-326357 and 10-151770, they depend only on a variation of the ejection efficiency caused by variation in environmental temperature, and these techniques cannot handle the variation of the pressure wave natural oscillation period Tc in accordance with environmental temperature. That is, using only these conventional techniques, it is impossible to prevent variation of ejection characteristics caused by environmental temperature in the liquid drop ejecting head which ejects high viscosity liquid, and to realize stable ejection.